Fix overflow in error emitter

compiler/rustc_errors/src/emitter.rs

@@ -2303,22 +2303,25 @@ impl EmitterWriter {
 
         // Colorize addition/replacements with green.
         for &SubstitutionHighlight { start, end } in highlight_parts {
-            // Account for tabs when highlighting (#87972).
-            let tabs: usize = line_to_add
-                .chars()
-                .take(start)
-                .map(|ch| match ch {
-                    '\t' => 3,
-                    _ => 0,
-                })
-                .sum();
-            buffer.set_style_range(
-                *row_num,
-                max_line_num_len + 3 + start + tabs,
-                max_line_num_len + 3 + end + tabs,
-                Style::Addition,
-                true,
-            );
+            // This is a no-op for empty ranges
+            if start != end {
+                // Account for tabs when highlighting (#87972).
+                let tabs: usize = line_to_add
+                    .chars()
+                    .take(start)
+                    .map(|ch| match ch {
+                        '\t' => 3,
+                        _ => 0,
+                    })
+                    .sum();
+                buffer.set_style_range(
+                    *row_num,
+                    max_line_num_len + 3 + start + tabs,
+                    max_line_num_len + 3 + end + tabs,
+                    Style::Addition,
+                    true,
+                );
+            }
         }
         *row_num += 1;
     }